Applied electromagnetic fields can be used in many diagnostic and therapeutic purposes. In such cases, electromagnetic fields are applied to biological tissue. Depending on the magnitude and duration of the power used, the fields can have different effects on the tissues. For example, low magnitude and extended duration of an electric potential can be used to drive charged therapeutic molecules through tissues in iontophoretic methods. High power electromagnetic fields in the radio and microwave frequencies can be used for tissue ablation to treat neoplastic diseases such as cancer. Energy packed in pulses with predominantly low frequency components can be used in combination with therapeutic molecules in electroporation and electropermeabilization. Pulses with components of higher magnitude and frequency, used in combination with therapeutic molecules or not, can be used in irreversible electroporation. Electromagnetic fields can also be used to excite tissues. In cardiac applications, electrodes can be used to apply enough power to excite heart tissues so that they are extrinsically activated, as in the case of cardiac pacemakers and defibrillators. Tissue excitation fields using lower power can also be used to excite tissues in the central and peripheral nervous tissue.
Although electromagnetic fields have been used in the above-noted therapeutic applications to positive effect, there is room for improvement. In many cases, the highest intensity of an applied field will be located in the tissue immediately proximate to the electrodes that are used to apply the field, which may not be the intended target for the energy. It would desirable if the spatial locations at which an electromagnetic field acts on a medium could be better controlled.